1. Field of the Invention
The present invention is related to a sensor for simultaneously measuring both normal and shear forces applied to the sensor, and further a statically responsive sensor for measuring shear forces. The present invention further includes a method of designing an object or a device using these sensors.
2. Technical Background
The need for a thin, flexible shear force sensor has been apparent in multiple fields of discipline. The ability to measure shear forces is needed in many applications including prevention of pressure sores, neural prosthesis feedback and in prosthetic limbs. No product has been successfully engineered to replace the shear force sensors naturally found in our skin. These natural sensors provide vital information back to the brain for both safety and control applications. These safety concerns include the prevention of excessive shear forces, which have been correlated with the formation of pressure ulcers. The control applications include providing sensory feedback for both gross and fine motor control.
There are three somewhat crude sensors or techniques that have been developed to measure shear forces to date, but each of these sensors have their own limitations. The most common shear sensor currently produced is based on the principle of a strange gauge. The sensor is made out of metal and must be fixed to the object of interest. Even if this sensor was fabricated from a flexible material and bonded to the surface where lateral movement was being measured, which would be difficult, the basic principle of this type of sensor is to measure strain, and therefore tension or compression due to the surface becoming longer or shorter (e.g., in bending) and does not measure uniform shear stresses whereby the entire surface shifts equally with respect to the underlying material. Another type of shear sensor has been developed using piezoelectric transducers. These sensors are embedded with piezoelectric strips within a device, which is then dragged along a surface. As the piezoelectric device is dragged along a surface the piezoelectric sensors indicate changes in shear forces and therefore changes in the surface features. The piezoelectric device, however, does not have a static response and therefore can't be used to detect and measure the static shear forces that are of most concern. Finally, there are capacitive based sensors. These capacitive based sensors use relatively stiff sheet and metallic conductors that act as a mechanical low-pass filter, effectively distributing the force-induced strains among a number of adjacent sites.
The shear sensors outlined above are interesting, but are not applicable for measurement of the shear forces at all locations around a human body or in many other applications. A small flexible and compliant shear force sensor is still needed. It is an object of this invention to provide such a statically responsive shear sensor. It is a further object of this invention to provide a flexible sensor for simultaneously measuring both normal and shear forces. It is still a further object of this invention to provide a method of designing an object or device using these types of sensors.